There is a dedicated maker of brass band uniforms which has been operating for donkeys years. They used to be called Brayshaws but were bought out by a company called Handley. I'm sure they could sort you out.

However, many bands nowadays are looking elsewhere because of the cost of Handley's British-made stuff. If it's cheaper stuff you're after, you might try an office/catering supplies company. Viking Direct often have work clothing in their catalogue and might be worth a look.

I can highly recommend Get Shirty. When I was at the Scottish Co-op (about eight years ago) we changed from traditional jackets to shirts and waistcoats. At that time we bought all new waistcoats and shirts (plus extras) for the price of three stage jackets.